(1) Field of the Invention
The present invention relates to a fixing device based on electromagnetic induction heating and an image forming apparatus provided with the fixing device.
(2) Description of the Related Art
Image forming apparatuses such as printers are provided with a fixing device that fixes an unfixed image, formed on a sheet and constituted by toner and the like, to the sheet by heating and applying pressure to the unfixed image when passing the sheet through a fixing nip. The fixing device may, for example, be based on electromagnetic induction heating. In such a fixing device, an excitation coil is provided on the outside of the running path of a fixing belt that has an induction heating layer. Magnetic flux that is generated by allowing alternating current to flow through the excitation coil is channeled to the induction heating layer, thereby heating the fixing belt.
In a fixing device based on electromagnetic induction heating, the heat capacity of the fixing belt can be set to a small value, thus allowing for a reduction in the time required for the temperature of the fixing belt to rise to a predetermined fixing temperature (i.e. the warm-up period).
As the heat capacity of the fixing belt decreases, however, the rate of temperature increase per unit of input power increases. Therefore, continuous use of small, narrow sheets leads to the problem that, compared to a region of the fixing belt through which the sheet passes (corresponding to the width of the sheet, and hereinafter referred to as the “sheet conveyance region”), the temperature rises in a region through which the sheet does not pass (hereinafter referred to as the “non-sheet conveyance region”) on either side of the sheet conveyance region in the direction of width of the belt. This leads to thermal destruction and deterioration of surrounding components.
One method for controlling a rise in temperature of the non-sheet conveyance region is to provide a self-adjusting temperature control function that reduces the amount of heat in the non-sheet conveyance region. With this method, a plate member (hereinafter referred to as a “heat-control plate”) is provided on the inside of the running path of the fixing belt, so that the fixing belt is between the excitation coil and the heat-control plate. The heat control plate includes a magnetic shunt alloy layer having a Curie point of a predetermined temperature higher than the fixing temperature. When the temperature of the non-sheet conveyance region rises to the Curie point, which is higher than the fixing temperature, the portion of the magnetic shunt alloy layer in the heat-control plate corresponding to the non-sheet conveyance region loses its magnetism.
The heat-control plate is in sliding contact with the inner circumferential surface of the fixing belt during rotation of the fixing belt and receives the load in the circumferential direction of the frictional force generated between the heat-control plate and the fixing belt. So that the heat-control plate does not change shape due to this load, the heat-control plate is strengthened by increasing the thickness of the heat-control plate, and by extending the edges of the heat-control plate in the direction of width of the belt beyond the edges of the fixing belt and securing the heat-control plate to the housing of the fixing device.
Increasing the thickness of the heat-control plate, however, causes a corresponding increase in the heat capacity of the heat-control plate, thus lowering the rate of temperature increase of the heat-control plate. This facilitates thermal transfer from the fixing belt to the heat-control plate, which in turn allows heat to escape to the device housing, thereby reducing the capability of the belt to rise in temperature.